Myoglobin is a monomeric, cytoplasmic hemoprotein expressed exclusively in cardiomyocytes and oxidative skeletal myofibers. The current paradigm proposes that myoglobin's sole functional role is to mediate oxygen transport within the heart in order to maintain oxidative phosphorylation for myocardial contractility. However, additional functions for myoglobin have been proposed including the role as a cytoprotective protein against reactive oxygen species and as a modulator of nitric oxide (NO). Our laboratory has engineered myoglobin knockout (Mb-/-) mice that are viable and have preserved cardiac function under normoxic conditions due to various cellular and molecular adaptations. We have shown that under chronic hypoxic conditions the Mb-/-mice develop left ventricular (LV) systolic dysfunction due to a NO-mediated, cGMP-independent mechanism. We have also demonstrated that over-expression of myoglobin in the mouse heart confers resistance to ischemic injury. Our overall hypothesis is that myoglobin serves cardioprotective roles in the heart by facilitating oxygen transport and regulating nitric oxide homeostasis within the cardiomyocyte. We will use our genetically modified mouse models to more fully characterize the regulation and function of myoglobin in the heart. Since evidence from human studies suggests there may be dysregulation of myoglobin expression in the failing human heart, our ultimate goal is to determine if augmentation of myoglobin expression and activity can be of therapeutic benefit in heart failure. To enhance our understanding of the regulation and functional roles of myoglobin in the heart, we propose the following three specific aims: 1) To define the mechanism underlying hypoxia-induced LV systolic dysfunction in the myoglobin null mice. 2) To define the cardioprotective mechanism(s) of myoglobin in the heart. 3) To define the transcriptional regulation of myoglobin gene expression under hypoxic conditions. The proposed experiments are hypothesis-driven and will determine the transcriptional regulation and functional roles of myoglobin in the heart. Ultimately, an enhanced understanding of myoglobin biology will provide opportunities for the development of new therapeutic measures in the treatment of patients with advanced heart failure.